Substation circuit



Dec. 25, 1945. A. E. WOODRUFF I SUBSTA'I'ION CIRCUIT Filed May 29, 1944 INVENTORJ ALBERT E. WOODRUFF ATTORNEY Patented Dec. 25, 1945 SUBSTATION CIRCUIT Albert E. Woodrufi, Oak Park, 111., assignor to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application May 29, 1944, Serial No. 537,913

7 Claims.

The present invention relates to substation telephone circuits in general but more particularly to portable field telephones and in which the portable substation circuit may be connected to lines varying greatly in length.

High efiiciency telephones designed for use on lines of high loss usually must be used also on short lines of low loss. Several difficulties are encountered when high emciency telephones are so used. On low loss lines the loudness level is so high that it is painful to the ear of the listener. Intelligibility is less than it would be if the loudness were less. There is howling in case of the handset either on long or short lines and furthermore, the side tone is high. The speaker hearing his own voice very loud tends to speak less loudly. The high level side tone tends to defeat the purpose of the high efficiency telephone. It is highly desirable to be able to adjust the efficiency of a high efliciency telephone and to reduce the side tone.

Eflicient sound translating devices have been used in a battery-less telephone system wherein no batteries are needed for transmission or receipt of telephone messages. A magnetic or sound powered receiver used in this type of system is shown in the Woodrufi et al. Patent 2,241,105.. It has been found that when a sound powered receiver, such as disclosed in Patent 2,241,105, is substituted for a regular telephone receiver in a telephone system using battery for talking purposes, the output level of the sound powered receiver is from 12 to 15 decibels higher thanthe regular receiver. Due to this high output level, this type of receiver can be used on longer lines than the regular receiver. output level of the receiver is too high and is annoying unless of course the user is partially deaf. For ordinary use on short lines it is necessary to reduce the output level of the sound powered receiver and the method used in the present invention is to include a suitable resistor in series with the receiver on short lines and to shunt this resistor on long lines. A switching key is provided on the handle of the handset between the transmitter and receiver for switching the resistor in and out of the receiver circuit.

. The principal object of the present invention is the provision of means for including a resistor in the receiver circuit of a high eificiency receiver to reduce the output level of the receiver on short lines in combination with a manually operable switching means for shunting out the resistor to increase the output level of the sound powered receiver when used on long lines.

On short lines the Another object of the invention is the provision of means for preventing howling and side tone whenever the transmitter is switched into the line circuit and this is accomplished by the transmitter switch including the resistor in the receiver circuit during talking to out down the energy thereby reducing the apparent efficiency of the receiver to a point where it will no longer how] and also to effectively reduce side tone.

These and other objects will be more readily understood by reference to the following detailed description and the associated drawing in which is diagrammatically represented a portable telephone substation circuit of the local battery type. Referring now to the drawing, the substation circuit comprises the hand generator G, ringer l0, auto-transformer windings I2, l3 and M, the battery B, and the handset including the transmitter T, receiver R, transmitter switch 6 and 5 and the volume control receiver switch 3 interconnected to form a local battery substation circuit of the anti-side tone type. The auto-transformer comseries with the transmitter key 6 are connected in shunt of the primary winding M of the autotransformer and the receiver including the volume control receiver switch 3 and resistor 1 are shunted by the secondary winding 12 of the auto-transformer and condenser 9. The ringer I0 is bridged across the line conductors I and 2 for giving an audible indication on incoming calls. The hand generator G and generator switch 20 are for use in transmitting signalling current over the line conductors on outgoing calls,

The secondary winding 13 of the auto-transformer is inductively coupled to the primary winding i4 and the secondary winding 12 is also inductively coupled to the primary winding M. The winding 12 has a number of turns of fine wire forming a relatively high resistance winding which together with the condenser 9 forms a balancing circuit, such circuit being commonly referred to as the artificial line.

Considering the action of the substation during transmission, the transmitter thumb switch 6 is also closed thereby completing a local direct current circuit from one terminal of battery through the transmitter T, the switch 6, primary windin M to the other terminal of battery. The contacts 5 are also opened when the transmitter thumb switch is operated to open the shunt around the resistor 'l. The action of the voice on the transmitter causes it to vary the direct current through the transmitter circuit to change it into undulating current, which upon flowing through the winding I4, induces in the windings I2 and I3 an alternating current of proportional strength. The auto-transformer action of the windings I2 and I3 amplifies the variations in current strength produced in the winding II. This current flows over two circuits, one over line conductor I through the external line circuit to line conductor 2, condenser I5, to the resistor I, the receiver coil winding of receiver R, coils I4 and I3; and the other over winding I2 of the auto-transformer, condenser 9, to the resistor I, the receiver coil winding back to winding I2. It will be noticed that the receiver is included in both these circuits and that the current flow in these two circuits are in D- position. If the artificial line, previously mentioned as being built in the winding I2, is equivalent to the external circuit and if the number of turns in windin I2 is equivalent to the windings I3 and I 4, the two ends of the receiver will be at the same potential and no current will flow through the receiver R, thereby providing the well-known anti-side tone circuit. This type of anti-side tone circuit is analogous to the wellknown Edison three wire circuit in which the windings I3 and I4 act as one generator and the winding I2 acts as a second generator and in which the receiver is included in the neutral wire and is connected at one end to the junction point of the artificial line and line load and at the other end to the junction point of the two generators connected in series. As is wellknown, if the line load and artificial line load are equal no current will flow over the neutral wire through the receiver.

Considering now the action of the substation during receipt of talking current and also assuming that the volume control switch 3 is operated and springs 5 are closed, a shunt circuit is closed around resistor I. During the time that the substation is receiving incoming signals an E. M. F. will be present across the line conductors I and 2, this E. M. F. being alternating and proportional to the strength of the signal that is being received. The current flowing in over conductor 2 and condenser I5 tends to divide into two paths, the one through the artificial line comprising condenser 9 and the winding I2 of the autotransiormer and the other path including springs 3 and 5, the winding 8 of the receiver R, the windings I4 and I3 of the auto-transformer, line conductor I and thence back to the external circuit. The current flowing through the windings I3 and I4 induces in the winding I2 an electromotive force in such a direction as to oppose the flow of current through the artificial line. The transmitter circuit at springs 6 may be opened or closed with very little effect in either position but with the springs closed the transmitter is connected in parallel with the winding I4 thereby providing a parallel path for some of the alternating current during receiving. This arrangement introduces a certain loss which is unavoidable and not particularly serious. From the foregoing it will be seen that a majority of the incoming signals traverse the receiver coil winding and are effected very little by the shunt path through the condenser 9 and winding I2.

' As previously stated the type of receiver used in this circuit is of the high efliciency type such as shown in Patent 2,241,105 and when the impedance in the entire coil winding of the receiver matches the impedance looking toward the line from the points 30 and 3| the maximum transference of energy to the receiver is attained. On long lines, with the resistor I shunted, the output level of this receiver when used in this circuit was found to be very satisfactory but when this circuit was used on short lines the output level of this receiver was too high and annoying. In order to reduce the output level of the receiver the resistor 'I is included in series with the receiver Winding in this circuit to introduce approximately 12 decibel loss. As shown in the drawing a volume control receiver switch 3 is provided in the handle of the handset to enable the user to at will include or exclude the resistor 1 in series with the receiver winding. Since the circuit is designed so that the impedance of the receiver winding matches the impedance when looking toward the line from points 30 and 3|, maximum transference of energy to the receiver is attained with the volume control receiver switch 3 operated. When this circuit is connected to a short line the output level of the receiver is too high for ordinary use and the user will open the switch 3 to thereby include the resistor I in series with the receiver winding in the circuit. The inclusion of the resistor 1 in series with the receiver winding 8 reduces the energy in this circuit with the result that approximately a 12 decibel loss is attained thereby bringing the output level of the receiver to approximately the same level as in an ordinary telephone receiver.

In spite of the reduction in side tone when a high efficiency receiver is used, as previously mentioned, there is still howling for frequencies for other than the frequency at which the anti-sidetone circuit is balanced. In order to prevent this howling, or reduce it to a negligible amount, it is necessary to reduce the high efliciency of the receiver when the transmitter is included in the circuit. In order to accomplish this a set of closed springs 5, normally included in the shunt circuit forresistor I, is provided and these springs 5 are opened whenever the transmitter thumb switch is operated to include the resistor I in the receiver circuit to thereby cut down the energy in the receiver circuit to prevent howling. As will be noted it is immaterial whether or not the receiver volume control switch 3 is operated as the opening of springs 5 removes the shortcircuit from around the resistor 1, thereby including this resistor in series. with the receiver in the receiver circuit.

What is claimed is:

Ina substation circuit, a line, a transformer having three windings, a handset including a transmitter and a receiver, a volume control receiver switch and a transmitter key mounted on the handle of said handset, a first receiving circuit including said volume control receiver switch, said transmitter key and said receiver connected in series with the first and second windings of said transformer and the conductors of said line. a second receiving circuit including said receiver and a resistance for reducing the energy in said receiver connected in series with said first and second windings of said transformer and the conductors of said line, a third winding of said transformer and a condenser in serial relation connected in shunt of said receiver to form an antiside tone circuit, a transmitter circuit including said key and transmitter in serial relation connected in shunt of said second winding of said transformer, said key providing means for at will opening and closing said transmitter circuit, means including said volume control receiver switch operative in accordance with the desired loudness of the receiver output for at will connecting either said first receiving circuit or said second receiving circuit to said line, and means comprising a pair of springs controlled by said transmitter key included in said first receiving circuit.

2. In a substation circuit, a line, a transformer having three windings, a handset including a transmitter and a receiver, a volume control receiver switch and a transmitter key mounted on the handle of said handset, a first receiving circuit including said volume control receiver switch, said transmitter key and said receiver connected in series with the first and second windings of said transformer and the conductors of said line, a second receiving circuit including said receiver and a resistance for reducing the energy in said receiver connected in series with said first and second windings of said transformer and the conductors of said line, a third winding of said transformer and a condenser in serial relation connected in shunt of said receiver to form an anti-side tone circuit, a transmitter circuit iricluding said key and transmitter in serial relation connected in shunt of said second winding of said transformer, said key providing means for at Will opening and closing said transmitter circuit, means including said volume control receiver switch operative in accordance with the desired loudness of the receiver output for at will connecting either said first receiving circuit or said second receiving circuit to said line, and means including a pair of springs controlled by said transmitter key for efiectively connecting said second receiving circuit to said line irrespective of the operated condition of said volume control receiver switch.

3. In a substation circuit for connection with a line having either a high transmission loss or a low transmission loss, a transformer having three windings, a handset including a transmitter and a receiver, a volume control receiver switch and a transmitter key mounted on the handle of the handset, a first receiving circuit for use with said line in case said line is a long line of high transmission loss, said first receiving circuit including said volume control receiver switch, a first portion of said transmitter key and a receiver connected in series with the first and second windings of said transformer and the conductors of said line; a second receiving circuit for use with said line in case said line is a short line of low transmission loss, said second receiving circuit including said receiver and a resistance for reducing the energy in said receiver connected in series with said first and second Windings of said transformer and the conductors of said line, a third winding of said transformer and a condenser in serial relation connected in shunt of said receiver to form an anti-side tone circuit, a transmitter circuit including another portion of said transmitter key and said transmitter in serial relation connected in shunt of said second winding of said transformer, means including said volume control receiver switch operative in accordance with the desired loudness of the receiver output for at will connecting either said first receiving circuit or said second receiving circuit to said line, means including said other portion of said transmitter key for at will opening and closing said transmitter circuit, and means including said first portion of said transmitter key connected in bridge of said resistance for switch ing said line to said first receiving circuit or to said second receiving circuit dependent upon the operated condition of said transmitter key.

4. In a substation circuit for connection with a line having either a high transmission loss or a low transmission loss, a transformer having three windings, a receiving circuit including a resistance and a receiver connected in series with the first and second windings of said transformer and the conductors of said line, a third winding of said transformer and a condenser in serial relation connected in shunt of said receiver forming an anti-side tone circuit, a transmitter cir cuit including a transmitter and a local battery in series connected in shunt of said second winding of said transformer, means including a transmitter key for at will closing and opening said transmitter circuit, a circuit for short-circuiting said resistance to increase the energy in said receiver in case said line is a long line of high transmission loss, and a volume control receiver switch and normally closed springs on said transmitter key connected in series for completing said shortcircuit around said resistance, said normally closed transmitter key springs opened in response to the operation of the transmitter key to include said resistance in said receiving circuit irrespective of the operated condition of said volume control receiver switch to reduce the energy in said receiver to prevent howling.

5. In a substation circuit for connecting with a line having either high transmission loss or a low transmission loss, a transformer having three windings, a receiving circuit including a receiver connected in series with the first and second windings of said transformer and the conductors of said line, a third winding of said transformer and a condenser in serial relation connected in shunt of said receiver forming an anti-side tone circuit, a transmitter circuit including a transmitter and a local battery in series connected in shunt of said second winding of said transformer, means including a transmitter key for at will opening and closing said transmitter circuit, a resistance connected to one of said line conductors and in series with said receiver, a circuit for short-circuiting said resistance to transmit the full energy of incoming signals transmitted over said line to said receiver, a volume control receiver switch operative for completing said short-circuit of said resistance in case said line is a long line of high transmission loss or operative for opening said short circuit to in-' clude said resistance in series with said receiver in said receiving circuit to reduce the energy of the incoming signals transmitted over said line to said receiver in case said line is a short line of low transmission loss, and normally closed springs controlled by said transmitter key for opening said short circuit to include said resistance in said receiving circuit during transmission of outgoing signals from said transmitter to reduce the energy in said receiver to prevent howling irrespective of the operated condition of said volume control receiver switch.

6, In a substation circuit for connecting with a line having either high transmission loss or a low transmission loss, a transformer having two windings, a receiving circuit including a receiver connected in series with the first winding of said transformer and the conductors of said line, a transmitter circuit including a transmitter and a local battery in series connected in shunt of said second winding of said transformer, means including a transmitter key for at will opening and closing said transmitter circuit, a resistance connected in series with said receiver and to one of said line conductors, a circuit for shortcircuiting said resistance to transmit the full energy of incoming signals transmitted over said line to said receiver, a volume control receiver switch operative for at will opening said short circuit to include said resistance in series with said receiver in the receiving circuit to reduce the energy of the incoming signals transmitted over said line to said receiver in case said line is a short line of low transmission loss, and normally closed springs controlled by said transmitter key for opening said short circuit to include said resistance in said receiving circuit during transmission of outgoing signals from said transmitter to reduce the energy in said receiver to prevent howling irrespective of the operated condition of said volume control receiver switch.

7. In a substation circuit, a line, a transformer having three windings and a condenser connected in series in bridge of the line conductors of said line, a transmitter connected in bridge of one of said windings, a high efficlency receiver connected in bridge of said condenser and another of said windings, said receiver having a winding whose impedance matches the normal impedance of said substation circuit at the points where said receiver is connected into said substation circuit to secure the greatest output level from said receiver in response to incoming signals, a resistance, a circuit for including said resistance in series with said receiver to reduce the energy of said incoming signals in said receiver to introduce a loss in the output level from said receiver in response to incoming signals of the same strength, a switching key for at will short-circuiting said resistance dependent upon the output level desired from said receiver, and a transmitter key for at will completing the connection of said transmitter in bridge of said one winding and for opening the said short circuit around said resistance in case the same is closed by said switching key.

ALBERT E. WOODRUFF. 

